1. Field of the Invention
The present invention relates to a speech coding apparatus and, more particularly, to a speech coding apparatus which operates with a high quality speech coding method.
By using a speech coding apparatus which operates with a high quality speech coding method, the following three advantages can be obtained in a digital communication system:
a) In general, using this method it is possible to band compress a digital speech signal transmitted at 64 kbps to, for example, 8 kbps, and, it is possible to transmit the digital speech signal at a very low bit rate. This can be a factor for reducing the so-called line transmission costs.
b) It becomes easy to simultaneously transmit speech signals and nonspeech signals (data signals). Therefore, there is a greater economic merit to the communication system and much greater convenience to the user.
c) When the transmission line making up the transmission system is a wireless transmission line, the radio frequency can be used much more efficiently and, in a communication system provided with a speech storage memory, a greater amount of speech data can be stored with the same memory capacity of the speech storage memory as before compression.
With the above-mentioned three advantages, the speech coding apparatus with a high quality speech coding method can be expected to be useful for the following systems:
1) Intraoffice digital communication systems,
2) Digital mobile radio communication systems (digital car telephones),
3) Speech data storage and response systems.
In this case, in a speech coding apparatus used for the communication systems of the above 1) and 2), it becomes important that, first, real time processing is possible and, second, the apparatus be constructed compactly.
2. Description of the Related Art
There are human operators on both the transmission side and reception side of a speech communication system. That is, signals expressing human speech (speech signals) serve as the medium for communication. These speech signals, as is known, include considerable redundancy. Redundancy means that there is a correlation between adjacent speech samples and also between samples separated by some periodic duration. If one takes into account this redundancy, when transmitting or storing speech signals, it becomes possible to reproduce speech signals of a sufficiently good quality even without transmitting or storing completely all the speech signals. Based on this observation, it is possible to remove the above-mentioned redundancy from the speech signals and compress the speech signals for greater efficiency. This is what is referred to as a high quality speech coding method. Research is proceeding in different countries on this at the present time.
Various forms of this high quality speech coding method have been proposed. One of these is the "code-excited linear prediction" speech coding method (hereinafter referred to as the CELP method). This CELP method is known as a very low bit rate speech coding method. Despite the very low bit rate, it is possible to reproduce speech signals with an extremely good quality.
Details of the conventional speech coding apparatus based on the CELP method will be given later, but note that there is a very grave problem involved with this method. The problem is the massive amount of digital calculations required for encoding speech. Therefore, it becomes extremely difficult to perform speech communication in real time. Theoretically, realization of such a speech coding apparatus enabling real time speech communication is possible, but a supercomputer would be required for the above digital calculations. This being so, it would be impossible to make a compact (handy type) speech coding apparatus in practice.